Split sheave, belt-driven continuously variable transmissions (CVT's) are used in a variety of recreational type off-road vehicles such as snowmobiles, golf carts, all-terrain vehicles (ATV's), and the like. CVT's, as their name implies, do not require shifting through a series of forward gears, but rather provide a continuously variable gear ratio that automatically adjusts as the vehicle speeds up or slows down, thus providing relatively easy operation for the rider.
A typical CVT transmission is made up of a split sheave primary drive clutch connected to the output of the vehicle engine (often the crankshaft) and split sheave secondary driven clutch connected (often through additional drive train linkages) to the vehicle axle. An endless, flexible, generally V-shaped drive belt is disposed about the clutches. Each of the clutches has a pair of complementary sheaves, one of the sheaves being movable with respect to the other. The effective gear ratio of the transmission is determined by the positions of the movable sheaves in each of the clutches. The primary drive clutch has its sheaves normally biased apart (e.g., by a coil spring), so that when the engine is at idle speeds, the drive belt does not effectively engage the sheaves, thereby conveying essentially no driving force to the secondary driven clutch. The secondary driven clutch has its sheaves normally biased together (e.g., by a torsion spring working in combination with a helix-type cam, as described below, so that when the engine is at idle speeds the drive belt rides near the outer perimeter of the driven clutch sheaves.
The spacing of the sheaves in the primary drive clutch usually is controlled by centrifugal flyweights. Centrifugal flyweights are typically connected to the engine shaft so that they rotate along with the engine shaft. As the engine shaft rotates faster (in response to increased engine speed) the flyweights also rotate faster and pivot outwardly, urging the movable sheave toward the stationary sheave. The more outwardly the flyweights pivot, the more the moveable sheave is moved toward the stationary sheave. This pinches the drive belt, causing the belt to begin rotating with the drive clutch, the belt in turn causing the driven clutch to begin to rotate. Further movement of the device clutch's movable sheave toward the stationary sheave forces the belt to climb outwardly on the drive clutch sheaves, increasing the effective diameter of the drive belt path around the drive clutch. Thus, the spacing of the sheaves in the drive clutch changes based on engine speed. The drive clutch therefore can be said to be speed sensitive.
As the sheaves of the drive clutch pinch the drive belt and force the belt to climb outwardly on the drive clutch sheaves, the belt (not being relatively stretchable) is pulled inwardly between the sheaves of the driven clutch, decreasing the effective diameter of the drive belt path around the driven clutch. This movement of the belt outwardly and inwardly on the drive and driven clutches, respectively, smoothly changes the effective gear ratio of the transmission in variable increments.
Split-sheave, belt driven CVTs are typically purely mechanical devices, that is, the mechanical parameters are established when the CVT is assembled. Once the CVT is assembled, the gear ratio depends on these set mechanical parameters. For example, the gear ratio depends on the distance between the drive clutch sheaves. The distance between the drive clutch sheaves is determined by the amount of force produced by the flyweights against the movable sheave. As the flyweights are attached to the engine shaft, the amount of the flyweight force depends on the speed of rotation of the engine shaft. Thus, with these prior devices, it is difficult to modify the gear ratio without disassembling the CVT and readjusting the mechanical parameters.
There are many situations in which it would be desirable to be able to easily modify the gear ratio of a CVT. It would be particularly desirable to be able to modify the gear ratio of a CVT during the actual operation of the CVT, to fit variable operating conditions. In some operating conditions, the mechanical gear ratio is less than optimal. For example, when the vehicle is traveling along at a given speed and then the rider momentarily lets off on the throttle, the centrifugal movement of the flyweights and other forces is disrupted, and can cause the system to momentarily shift out of the desired transmission ratio. When the rider then again applies the throttle, torque is restored to the driven clutch, but the transmission is no longer in its optimal gear ratio, and takes a moment to adjust. Similarly, if the drive wheels momentarily leave the ground (such as when a professional rider goes off a jump) but the rider does not let off on the throttle, the load on the drive wheels is momentarily substantially reduced, again disrupting the balance of forces within the CVT and causing it to temporarily shift out of the desired gear ratio. When load is restored to the drive wheels, the CVT must again readjust to the proper gear ratio. Thus, there are situations in which it would be desirable to modify the gear ratio independently of the mechanical parameters set by the CVT at the time of manufacture.